Voluminous research has linked nonresident fatherhood to a doubling of the risk for sexual outcomes in adolescence such as accelerated pubertal timing, earlier sexual debut, and risky sexual behavior. This research has sparked public concern because these outcomes elevate risk of sexually transmitted disease and teenage pregnancy, and because they increase the likelihood of nonmarital childbirth and family instability in adulthood. To design social policies that weaken this cycle, however, research must illuminate whether it is father absence or characteristics that select families into disruption tha drive these links, for the causal and selection hypotheses imply different processes by which these outcomes emerge and, thus, different policy approaches to reducing their prevalence. A handful of recent studies have estimated the influence of selection by comparing siblings and cousins who are discordant for nonresident fatherhood, either the experience of it or the duration of the experience, thereby minimizing the influence of unobserved environmental and genetic risks that vary between families. However, these studies examined only (girls') age at menarche and first intercourse, used relatively small samples of related pairs, and did not investigate mechanisms that may link nonresident fatherhood to sexual development or environmental factors that moderate those links. The proposed study would extend the use of within-family comparisons by examining a wider range of sexual outcomes, using larger and more genetically diverse samples of related pairs, investigating unique pathways of influence, and exploring father involvement and stepfather presence as moderators. It would also explore whether any selection effects are environmental or genetic in origin by comparing associations across adolescents with different levels of genetic relatedness. The project would use two of the only nationally representative datasets with sibling and cousin pairs and data on adolescent sexual development, Children of the National Longitudinal Survey of Youth - Young Adult Survey (CNLSY-YA) and the National Longitudinal Study of Adolescent Health (Add Health), to pursue the following aims: 1) estimate the extent to which selection drives links between nonresident fatherhood and adolescent sexual development and whether the selection process is environmental or genetic in origin; 2) illuminate mechanisms that may account for these links by comparing associations between nonresident fatherhood and sexual development across different sexual domains and comparing associations by the timing of father departure from the child's life; and 3) determine if father involvement or stepfather presence moderate links by comparing within-family effects across children and families with different qualities and quantities father involvement and different experiences of stepfather coresidence. Overall, this project aims to clarify the implications of nonresident fatherhood for adolescent sexual development, and in doing so, the implications of ever-increasing rates of nonresident fatherhood for family formation in next generation.